Light emitting diodes (LEDs) are currently contemplated as light sources for several different lighting applications, and the use of light emitting diodes is expected to grow in the coming years.
A light emitting diode is typically comprised in a package containing the actual LED-chip comprising the active, light producing, layers, and light extraction optics arranged on the LED-chip. The light extraction efficiency between the chips ant the optics of the package is a major issue with which LEDs are confronted.
A classical approach in this context involves the use of primary extraction optics, e.g. optical domes provided on the LED-chips, which optical domes extract the light based on their refractive properties. The materials of these optical domes are often based on silicones and polymers (such as PMMA). However, these optical domes have limited photo-thermal stability, which limits the power of the used LED-chips, which in turn limits the lumen power of the light-emitting device.
An approach to solve this is to use inorganic optical elements for the extraction of light from LED chips. The material of such optical elements can for example be polycrystalline ceramic materials or glass. Such inorganic optical elements have much higher photo-thermal stability, which allows light emitting devices with high lumen power and output.
However, high power LEDs may dissipate significant amounts of heat, and the radiation may be intense. In this context, the bond between the LED chip and the extraction optics, which forms a junction that couples light from the LED chip to the extraction optics and that physically binds the extraction optics to the LED chip, should itself exhibit high photo-thermal stability, so that it is not the limiting factor in the light-emitting device, and so that it is able to benefit from the high photo-thermal stability of the inorganic extraction optics.
However, as high power LEDs may dissipate a lot of heat in operation, slight differences between the coefficients of thermal expansion for the LED-chip, the bonding material and extraction optics results in build-up of heat induced tensions in the package, eventually leading to breakage of the package. Thus, there is a need for a light-emitting device where the bond between the LED-chip and the extraction optics can withstand the load and stress it is exposed to.